Method of producing permanent colored aluminum castings



United States Patent U.S. Cl. 204-33 4 Claims ABSTRACT OF THE DISCLOSURE Aluminum castings containing 80 to 95.5% aluminum and 4 /2 to 20% silicon together with a coloring metal are etched by a solution containing hydrofluoric acid for removal of the silicon from the surface of the castings. The castings are then anodized. The oxidized coloring metal impart permanent color to the castings.

This invention has to do with the production of permanently colored aluminum alloy castings. Aluminum silicon alloys, containing from approximately 4.5% to 20% silicon, are the most readily castable aluminum alloys known today. The outstanding casting characteristics of aluminum silicon alloys are due to their comparatively high fluidity in the molten state, excellent feeding properties during solidification, and comparative freedom from hot shortness. Other aluminum alloys, such as aluminum-magnesium, aluminum-zinc, aluminum-copper, etc., not containing any appreciable percentage of silicon, have inherent casting problems due either to relatively poor fluidity, poor feeding properties during solidification, or they are hot short (i.e. brittle). Castings from such alloys often contain imperfections, such as shrinkage porosity, cracks, and mis-runs, which reduce physical properties and produce a poor appearance, if the castings are anodized.

Aluminum castings are often surface treated to improve their appearance, corrosion resistance, and abrasion resistance. The principal method of such surface treating is by anodizing. The anodizing process is well known. It converts a thin surface film of the aluminum casting to aluminum oxide, Which adheres tightly to the surface of the aluminum casting, and protects the surface of the casting from corrosion. The anodized oxide coating is extremely hard and abrasion resistant. If the casting is formed of pure aluminum, the anodized oxide film varies from colorless to white, depending on the thickness of the film formed by the anodizing treatment. It is often desirable to impart colors to aluminum casting, which may be done by the addition of dyes in the anodizing bath. However, colors so formed are not stable and especially so if the castings are continuously subjected to the weather.

At present, aluminum castings have been used rather extensively in fabricating the exterior trim on buildings. In such use, the castings are, of course, continuously subject to the weather, and the dye coloring is short-lived.

In the use of castings for ornamental purposes, such as the exterior trim on a building, it is of course important that the castings be free of blow holes, cracks, or other defects which would detract from their appearance. Ac-

3,505,179 Patented Apr. 7, 1970 cordingly, as previously stated, it is desirable to make such castings from the aluminum silicon alloy.

If the castings are formed of aluminum silicon alloy, and are anodized, the result is that the silicon in the al oy is also oxidized, and the casting will have various shades of gray to practically black, depending on the amount of silicon contained in the alloy. Various other metals may also be included in the aluminum silicon alloy, and their presence will impart different colors to the castings when they are anodized. For example, additions of manganese will produce oxide films with brown, or bronze, shades. The addition of chromium to the alloy will produce a gold color. However, due to the presence of the silicon in the alloy, the silicon oxide formed in the anodizing treatment tends to mask out any other colors produced by metal oxides, such as manganese, chromium and magnesium.

This invention has as an object a method of producing aluminum silicon alloy castings having a desired color which is permanent and will not change even if the castings are continuously subjected to weathering.

According to my invention, the castings are formed from an alloy containing from to aluminum; 4 /2 to 20% silicon, and a coloring metal, such as magnesium 0 to 10%, or chromium 0 to .3%, or manganese 0 to 1%. The alloy casting is treated in an etching bath and subsequently anodized The etching bath is so constituted that the atoms of silicon present in the surface of the casting are removed without disturbing the atoms of the coloring metal, and without adversely affecting the aluminum in the alloy, with the result that when the casting is anodized, molecules of the oxide of aluminum are formed on the surface of the casting and are combined with the oxides of the coloring metal However, the oxides of silicon are not present in the surface of the casting when treated according to my invention because the atoms of silicon have been removed from the surface of the casting and, therefore, are not present to form oxides of silicon in the anodizing process. Accordingly, the oxides of the coloring metals are permitted to contribute their particular colors to the aluminum oxide coating formed by the anodizing treatment, whereby a pleasing color combination is imparted to the casting, which then has the properties of improved corrosion resistance and increased hardness typical of anodized aluminum castings.

A typical etching bath suitable for carrying out my invention, consists of- Parts by vol. Hydrofluoric acid Hydrochloric acid 5 Nitric acid 20 Water 950 3 4 1111111; 4 /2 to 20% silicon, and a coloring metal, etching References Cited 2231i snurface of the casting with an etching solution con- UNITED STATES PATENTS g ML 2,040,618 5/1936 Mason et a1 204-8 XR Hydrofluoric acid Pettit Hydrochloride acid 5 5 3,140,203 7/1964 Grunwald 156-22 XR fjij: 950 JOHN H. MACK, Primary Examiner and than anodizing the cast-mg. W. VAN SISE, Assistant Examiner 2. The method set forth in claim 1, wherein the alloy 10 US, Cl, X,R.

includes magnesium from up to 10%.

3. The method set forth in claim 1, wherein the coloring metal consists of chromium up to .3%

4. The method set forth in claim 1, wherein the coloring metal consists of manganese up to 1%. 

